Digging into nonclassical pathways to crystallization to unearth design principles for
fabricating advanced functionalized materials shapes the future of materials science. Nature
has long since been exploiting such nonclassical pathways to crystallization to build
inorganic-organic hybrid materials that fulfill support, mastication, defense, attack, or optical
functions. Especially, various biomineralizing taxa such as stony corals deposit metastable,
magnesium-rich, amorphous calcium carbonate nanoparticles that further transform into
higher-order mineral structures. Here we examine whether a similar process can be duplicate
in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate
nanoparticles. Applying a combination of ultrahigh-resolution imaging, and, in situ, solidstate nuclear magnetic resonance (NMR) spectroscopy, we reveal the underlying mechanism
of the phase transformation of these synthetic amorphous nanoparticles into crystals. When
soaked in water, these synthetic amorphous nanoparticles are coated by a rigid hydration layer
of bound water molecules. In addition, fast chemical exchanges occur between hydrogens
from the nanoparticles and those from the free water molecules of the surrounding aqueous
medium. At some stage, crystallization spontaneously occurs, and we provide spectroscopic
evidence for a solid-state phase transformation of the starting amorphous nanoparticles into
crystals. Depending on their initial chemical composition, and especially on the amount of
magnesium, the starting amorphous nanoparticles can aggregate and form ordered mineral
structures through crystal growth by particle attachment, or rather dissolve and reprecipitate
into another crystalline phase. The former scenario offers promising prospects for exerting
some control over such non-classical pathway to crystallization to design mineral structures
that could not be achieved through a classical layer-by-layer growth.<br>
Solid-State Phase Transformation and Self-Assembly of Amorphous Nanoparticles into Higher-Order Mineral Structures
